The present invention generally relates to a rotary type power hand tool having a rotational output shaft, and in particular to a locking device for fixing the output shaft during a power failure so as to allow the hand tool to be driven manually.
A conventional rotary type power hand tool is powered by an external power source, such as an electric main, or a built-in power source, such as a rechargeable battery set. A rotary type power hand tool comprises a rotational output shaft having an outer tip to which a chuck is mounted for gripping for example a drill bit or a screwdriver. In case of power failure for externally powered tools or insufficiency of power supply of an internally powered tool, the drill bit may get stuck in a work piece. To remove the drill bit from the work piece, a rotation of the drill bit in an opposite direction is usually performed manually. It is thus desired to drive the drill bit by manually rotating the power tool in case of power failure.
Conventionally, the output shaft is driven by a motor via a gear train. No means is provided for fixing the output shaft whereby when a user tries to drive the drill bit by manually rotating the hand tool, a relative rotation occurs between the output shaft and the hand tool, preventing the user to drive the drill bit by manually rotating the hand tool.
Thus, it is desired to provide a locking device for fixing the output shaft to overcome the above problem.
Accordingly, an object of the present invention is to provide a locking device for a rotary type power hand tool which fixes an output shaft of the hand tool so as to allow a user to drive the output shaft by manually rotating the hand tool.
In accordance with the present invention, there is provided a locking device adapted to be arranged between a gear train and an output shaft of a power hand tool. The locking device comprises a fixed ring fixed in a housing of the hand tool. A disk drivingly engages the output shaft to be rotatable about a first axis. Separate locking pawls each having a second axis are concentrically arranged around the first axis and supported by a pair of retention rings to orbit about the first axis. Each pawl has a V-shaped projection received in a corresponding V-shaped notch defined in the disk whereby the pawl is capable of a very limited rotation about the second axis. The pawls are arranged in a central bore of the fixed ring with an outer face thereof opposing an inner circumference of the central bore with a tiny gap therebetween. A coupler drivingly engages the shaft and the retention rings whereby when the coupler is rotated by the gear train, the coupler drives the disk and the pawls simultaneously and thus allowing the shaft to be rotated without constraint. When a user manually rotates the output shaft, the disk is rotated while the pawls are prevented from rotation about the first axis which causes a rotation of each pawl about the second axis leading to an interference between the outer face thereof and the inner circumference of the fixed ring thereby preventing the shaft from being further rotated.